Conventionally, composite oxides containing various metal oxides have been utilized as a support, a co-catalyst, and the like for a catalyst for purification of exhaust gas. As a metal oxide in such a composite oxide, ceria is favorably used because ceria is capable of storing and releasing oxygen according to the oxygen partial pressure in the atmosphere (i.e., has an oxygen storage capacity). In addition, recently, various kinds of composite oxides containing ceria have been studied, and there have been disclosed various ceria-zirconia base composite oxides and methods for producing the same.
For example, Japanese Unexamined Patent Application Publication No. Hei 8-109020 (Patent Literature 1) discloses a composite oxide comprising cerium oxide, zirconium oxide and hafnium oxide, in which the content of cerium oxide is 4.99 to 98.99% by mass, that of zirconium oxide is 1 to 95% by mass, and that of hafnium oxide is 0.01 to 20% by mass. The composite oxide includes p′ phase as a crystal phase. In addition, as a method for producing the composite oxide described in Patent Literature 1, there is disclosed a method in which a primary composite oxide containing cerium oxide, zirconium oxide and hafnium oxide is subjected to a reduction treatment at 600 to 1000° C. for 0.5 to 10 hours and then to a heating-oxidation treatment.
Moreover, Japanese Unexamined Patent Application Publication No. Hei 8-109021 (Patent Literature 2) discloses a composite oxide comprising cerium oxide, zirconium oxide and hafnium oxide, in which the content of cerium oxide is 4.99 to 98.89% by mass, that of zirconium oxide is 1 to 95% by mass, and that of hafnium oxide is 0.01 to 20% by mass, the composite oxide further comprising 0.1 to 10% by mass of titanium oxide, tungsten oxide, nickel oxide, copper oxide, iron oxide, aluminum oxide, silicon oxide, beryllium oxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide, an oxide of a rare earth metal other than cerium, or a mixture thereof. The composite oxide includes φ phase as a crystal phase. In addition, as a method for producing the composite oxide described in Patent Literature 2, there is disclosed a method in which cerium oxide, zirconium oxide and hafnium oxide are mixed with titanium oxide, tungsten oxide, nickel oxide, copper oxide, aluminum oxide, silicon oxide, beryllium oxide, magnesium oxide, calcium oxide, strontium oxide, barium oxide, an oxide of a rare earth metal other than cerium or a mixture thereof, followed by compression-molding and then calcination at 700 to 1500° C. (preferably 900 to 1300° C.) to form φ phase.
Further, Japanese Unexamined Patent Application Publication No. Hei 8-103650 (Patent Literature 3) discloses a method for producing a composite oxide, characterized in that an composite oxide containing cerium oxide, zirconium oxide and hafnium oxide as essential components is subjected to a heating-reduction treatment and then to a heating-oxidation treatment. As a favorable method for the heating-reduction treatment, there is disclosed a method including heating at 600 to 1000° C. for 0.5 to 10 hours in an atmosphere of a reducing gas.
Additionally, International Publication No. WO2006/030763 (Patent Literature 4) discloses a method for producing a cerium-zirconium base composite oxide, characterized by comprising: mixing a cerium raw material containing cerium oxide with a zirconium raw material containing zirconium oxide at a predetermined ratio to prepare a raw-material mixture in advance; melting the mixture at a temperature equal to or above the melting point; then cooling a melted product thus obtained to form an ingot; subsequently crushing the ingot to obtain a powder; removing strain in a crystal of the powder under heating as desired; and further pulverizing the powder.
Furthermore, “Study on Oxygen Storage/Release Performance and Crystal Structure of Ceria-Zirconia Compounds with Different Ce/Zr Ratios” (Non Patent Literature 1) in the thesis by Mr. Iwao Sasaki in 2004, pp. 150 to 170, and “Material Design and Crystal Structure Analysis Focusing on Orderly Arrangement of Ceria-Zirconia Solid Solution” described at page 140 in Proceedings of the 2006 Spring Meeting of the Japan Institute of Metals (Non Patent Literature 2) disclose a ceria-zirconia base composite oxide obtained by subjecting a ceria-zirconia base composite oxide prepared by a coprecipitation method to a reduction treatment at 1673K (1400° C.).
However, a ceria-zirconia base composite oxide obtained by adopting production methods as described in Patent Literatures 1 to 4 and Non Patent Literatures 1 to 2 above is not produced so as to enhance the durability in a high-temperature, oxidizing atmosphere. The heat resistance is not always sufficient. For this reason, such conventional ceria-zirconia base composite oxides do not always have sufficient oxygen storage capacity after exposure to high temperature for an extended period of time.
Furthermore, Japanese Unexamined Patent Application Publication No. 2009-84061 (Patent Literature 5) discloses a ceria-zirconia base composite oxide which has 500 or more of an orderly arranged pyrochlore-type phase remained after heating under a temperature condition of 1000° C. in air for hours. The ceria-zirconia base composite oxide is obtained by subjecting a ceria-zirconia base composite oxide powder having a content ratio between ceria and zirconia in a range from 55:45 to 49:51 in terms of molar ratio ([ceria]:[zirconia]) to a reduction treatment under a temperature condition from 1500° C. or higher to 1900° C. or lower and then to an oxidation treatment.